JPH01117072A - Signal transmitter - Google Patents

Abstract

PURPOSE:To prevent a malfunction due to noises by connecting a capacitor in parallel between a base and an emitter for an output transistor and charging and discharging the capacitor in response to the ON-OFF operation of a photo- detector. CONSTITUTION:When a phototransistor 102 is turned ON, a transmission signal is output after a fixed delay time from an output transistor 201. When the phototransistor 102 is turned OFF, the transmission signal is output at high speed without the delay time from the output transistor 201. Even when noises are overlapped between a power line and the phototransistor 102 is turned ON instantaneously, the output transistor 201 is left as it is under cut-off because sufficient charges are not stored in a capacitor 202, thus preventing the output of the transmission signal by mistake.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a signal transmission device that transmits on/off signals using a photocoupler.

<Prior Art> As a conventional signal transmission device of this type, there is one shown in FIG. This signal transmission device includes a photocoupler 100 and a signal output circuit 300. The photocoupler 100 includes a light emitting element (LED in this example) 101, a light receiving element (phototransistor in this example) 102, and a signal transmission device 3.
00 is composed of resistors 301 and 302 and an output transistor 303, respectively. Then, the signal output circuit 30
0 and the phototransistor 102 are connected to a DC power supply ±V, and the emitter of the phototransistor 102 is connected to the base of the output transistor 303.

In this signal transmission device, the LEDIO of the photocoupler 100
When a signal is input to I, the phototransistor 102 is turned on, and accordingly, the output transistor 303 is turned on and a low level transmission signal is output from its emitter. Further, when the phototransistor 102 is turned off, a high level transmission signal is output from the emitter of the output transistor 303.

FIG. 3(a) is a characteristic diagram showing the relationship between the collector-emitter voltage Vce of the phototransistor 102 and the switching time Tw, and FIG. FIG.

Here, the switching time Tw means the delay time of a signal output from the phototransistor 102, which is a light receiving element, in response to the on/off operation of the LED 101, which is a light emitting element of the photocoupler 100.

As can be seen from these figures, in order to shorten the switching time Tw of the photocoupler 100 and improve the signal response, it is necessary to increase the collector-emitter voltage Vce of the phototransistor 102 to a certain extent, and also increase the voltage between the phototransistor 102 and the DC power supply. It is necessary to reduce the resistance value of The signal transmission device shown in FIG. 2 satisfies the above conditions and can transmit signals at high speed.

<Problems to be Solved by the Invention> However, the conventional device having the above-mentioned configuration has a problem in that it tends to malfunction when noise is superimposed on the power supply line to which the phototransistor 102 is connected.

In other words, the sudden voltage change of noise (d V/d t
), even when the phototransistor 102 is in the off state, a weak current flows due to its junction capacitance. Moreover, since the current amplification factor h 2 of the phototransistor 102 is usually set to a large value, a minute current is amplified, and as a result, the output transistor 303 may turn on and the transmission signal may be erroneously output. This can be said to be a problem that inevitably arises because the phototransistor 102 used as a light receiving element has to have a structure with a sufficiently large amplification factor in order to amplify a weak photocurrent.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a signal transmission device that has short switching time, ensures high-speed signal transmission, and does not cause malfunctions due to noise. .

<Means for Solving the Problems> In order to achieve the above object, the present invention includes a photocoupler,
A signal transmission device including a signal output circuit having an output transistor that outputs a transmission signal corresponding to a human input signal to the photocoupler has the following configuration.

That is, the signal transmission device of the present invention includes a capacitor connected in parallel between the base and emitter of the output transistor, and a switching transistor that charges and discharges the capacitor in response to the on/off operation of the light receiving element. It is a feature.

Effect> In the signal transmission device of the present invention, the capacitor is charged when the photo-receiving element of the photocoupler is turned on, and only when the charging voltage reaches a predetermined value or higher, the output transistor becomes conductive and a transmission signal is output. Further, when the light receiving element is turned off, the capacitor is rapidly discharged, so that the output transistor immediately outputs a transmission signal. That is, when the light receiving element of the photocoupler is turned on, a transmission signal is outputted from the output transistor with a delay time, and when it is turned off, the transmission signal is outputted from the output transistor at high speed without delay.

Therefore, even if the light-receiving element of the photocoupler is momentarily turned on due to noise, the output transistor is maintained in a cut-off state, so that the transmission signal will not be erroneously output.

<Embodiment> FIG. 1 is a circuit diagram showing a signal transmission device according to an embodiment of the present invention. In the figure, reference numeral 100 denotes a photocoupler;
00 is a signal output circuit. The photocoupler 100 described above is composed of an LEDlol as a light emitting element and a phototransistor I02 as a light receiving element. The signal output circuit 200 also includes an output transistor 201 that outputs a transmission signal corresponding to an input signal to the photocoupler 100, a capacitor 202 connected in parallel between the base and emitter of the output transistor 201, and a phototransistor 102.
an eleventh second switching transistor 203.20 that charges and discharges the capacitor 202 in response to the on/off operation of the
4 and resistors 205 to 208, the signal output circuit 2
00 and the phototransistor 102 are connected to a DC power supply.

Next, the operation of the signal transmission device of the present invention will be explained.

In order to shorten the switching time of the phototransistor 102, when the phototransistor is off, its collector
A voltage lower than the base-emitter voltage V″io of the second switching transistor 204 by 5V is applied between the emitters, and the output transistor 201 is also turned off. The resistance value is also set small.

When a signal is input to the photocoupler 100, the LEDlol emits light, and the phototransistor 102 is accordingly turned on. Then, the second switching transistor 204 is immediately turned on and the first switching transistor 203 is turned off. However, the output transistor 201 becomes conductive only when the capacitor 202 is charged by the charge flowing through the resistor 206 and its voltage exceeds a threshold value. That is, when the phototransistor 102 is turned on, a transmission signal is output from the output transistor 201 with a certain delay time.

On the other hand, when the phototransistor 102 is turned off, the second switching transistor 204 is immediately turned off and the first switching transistor 203 is turned on. As a result, the charge accumulated in the capacitor 202 is rapidly discharged, so that the output transistor 201 is also turned off with almost no delay time. That is, when the phototransistor 102 is turned off, the transmission signal is output from the output transistor 201 at high speed without any delay time.

Furthermore, even if noise is superimposed between the power supply lines and the phototransistor 102 is momentarily turned on, the capacitor 2
02 does not accumulate enough charge to turn on the output transistor 201, so the output transistor 201
is maintained in the cutoff state. That is, due to the delay time of signal transmission by the signal output circuit 200, the noise appears as if it were filtered, and the transmission signal is not erroneously output.

Normally, in the case of synchronization signals and timing signals, only the rise or fall of the signal, i.e., turn-on or turn-off, is used as a trigger, and there are cases where it is only necessary to accurately transmit that timing. many.

In the signal transmission device of the present invention, as described above, since signals are transmitted at high speed when the phototransistor 102 is turned off, synchronization signals and timing signals can be transmitted at high speed at that timing.

In this embodiment, a case has been described in which the phototransistor 102 is used as the light receiving element of the photocoupler 100, but it goes without saying that similar effects can be obtained when a photodiode is used.

<Effects of the Invention> As described above, according to the present invention, switching time is short and high-speed signal transmission is ensured without any difference from the conventional technology. Moreover, excellent effects such as no malfunction due to noise are exhibited.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a signal transmission device according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional signal transmission device, and FIG.
The figure is a characteristic diagram for explaining the operation of a photocoupler. 100... Photocoupler, 101... Light emitting element (LE
D), 102... Light receiving element (phototransistor), 2
00...Signal output circuit, 201...Output transistor, 202...Capacitor, 203.204...Switching transistor.

Claims (1)

[Claims] (1) In a signal transmission device comprising a photocoupler and a signal output circuit having an output transistor that outputs a transmission signal corresponding to an input signal to the photocoupler, a capacitor is connected in parallel between the base and emitter of the output transistor. A signal transmission device further comprising a switching transistor that charges and discharges the capacitor in response to the on/off operation of the light receiving element.